Audio devices such as microphones, speakers, earphones, and headphones include: a transducing part that performs conversion between sound and an electrical signal; and a housing enclosing the transducing part. The transducing part includes an acoustic element, such as a vibration plate, which sends and/or receives sound. The acoustic element may be exposed to the outside of the housing as with the case of common speakers, or may be enclosed within the housing as with the case of earphones and microphones. When the acoustic element is enclosed within the housing, the housing is provided with a sound transmission port which is an opening for transmission of sound between the acoustic element and the outside of the housing.
The housing of an audio device is typically provided with an opening other than the sound transmission port, except when the housing is intentionally designed not to have such an opening. If the acoustic element is exposed to the outside of the housing but the housing itself is sealed, or if a space between the acoustic element and the sound transmission port is open to the outside through the sound transmission port but the opposite space in the housing is sealed, the pressure in the sealed space varies with the movement of the acoustic element. The pressure variation disturbs the vibration of the acoustic element and thereby deteriorates the sound output characteristics and/or sound input characteristics of the audio device (these characteristics may hereinafter be simply referred to as “audio device characteristics”), unless the audio device is delicately designed. The influence of the pressure variation is great when the volume of the sealed space is particularly small relative to the size of the acoustic element, such as in earphones. Providing the housing with an opening other than the sound transmission port can prevent such a sealed condition, leading to an improvement in the vibration characteristics of the acoustic element and therefore an improvement in the audio device characteristics.
In some audio devices, an acoustic resistor is disposed in an air passage between an acoustic element and housing openings including a sound transmission port. The acoustic resistor, although having air permeability, acts as an airflow resistor in the presence of which the air movement in the passage becomes more disturbed than in its absence. The placement of the acoustic resistor allows control of the air movement in the passage. Since sound is a vibration of air, the placement of the acoustic resistor between the acoustic element and the sound transmission port allows control of the characteristics of sound sent from the acoustic element and/or sound received by the acoustic element and therefore control of the audio device characteristics. In addition, the placement of the acoustic resistor between an opening other than the sound transmission port and the acoustic element allows control of air movement acting on the side of the acoustic element facing the opening, thus allowing control of the vibration of the acoustic element and therefore control of the characteristics of sound sent from the acoustic element and/or sound received by the acoustic element.
Patent Literatures 1 to 3 each disclose an audio device in which an acoustic resistor is placed. The acoustic resistors disclosed in these literatures are formed of a porous body such as a sponge, of a non-woven fabric, or of a woven fabric such as a mesh.